There has been increasing interest in the synthesis of crystalline films such as diamond films for use in coating applications and for applying to cutting and polishing tools. The development and synthesis of cubic boron nitride ("cBN") films has become of particular interest in recent years due to cBN being physically and structurally superior to diamond. cBN has a high relative hardness and wear resistance that is similar to diamond, but also has a high thermal stability in the presence of oxygen and a chemical inertness to ferrous materials, especially at higher temperatures which are not found in diamonds. Thus, cBN films have become highly desirable for use in coating applications that require exposure to high temperatures and highly oxidizing environments, as well as for applications to cutting and polishing tools for cutting ferrous materials and alloys due to their high thermal stability and chemical inertness to ferrous materials. In fact, bulk cBN is widely used for cutting tool inserts and as protective coatings to improve machining precision and to extend the life span of cutting and grinding tools significantly as well as for hard protective coatings for power electronic devices and optical systems for use in harsh environments. In addition, the high thermal conductivity of cBN also makes it an excellent material for use as a heat sink and for insulating materials.
cBN films are typically formed using deposition techniques for the deposition of thin films of boron nitride on a substrate. Such deposition techniques include physical vapor deposition techniques such as ion beam deposition, ion assisted pulsed laser deposition, and RF sputtering; and chemical vapor deposition techniques including plasma enhanced chemical vapor deposition, such as microwave electron cyclotron resonance, inductively coupled RF plasma enhanced chemical vapor deposition and hot filament enhanced chemical vapor deposition, and field emission electron enhanced chemical vapor deposition. Most of these deposition techniques, including RF sputtering, rely on energetic ion bombardment as a key for formation of cBN at low pressures. In such processes, atoms of boron and nitrogen are directed at a substrate while at the same time, the substrate is bombarded with ions to cause the boron and nitrogen atoms to reform on the substrate in a cubic boron nitride film.
With most of these synthesizing deposition techniques, however, it has been difficult to obtain pure or near 100% cubic phase boron nitride films using such techniques, and those that have produced high purity cubic phase boron nitride films have generally required the use of energetic ion bombardment of the substrate on which the cubic boron nitride film is deposited. The problem with such energetic ion bombardment of the substrate is that it limits the grain size of the cubic boron nitride films formed to nanocrystalline films which have a limited grain size. In addition, using ion bombardment creates a problem of resputtering of the boron and nitrogen atoms away from the substrate after a certain size film has been formed, further limiting the size and purity of the films. The ions bombarding the substrate and thus the boron nitride crystalline film generally have a relatively large mass and as they strike the film, the momentum transfer between the ions and the film reaches a point as the film is built up to a sufficient mass where the momentum transfer from the ions is sufficient to repel the boron and nitrogen atoms instead of attracting the boron and nitrogen atoms to the substrate.
As a result, large crystalline structure growth of cubic phase boron nitride films is inhibited and at some point stopped as the crystalline structure of the film reaches a certain size due to this resputtering effect of the ions striking the film. Thus, conventional synthesizing or deposition techniques for forming cubic phase boron nitride films that utilize ion bombardment generally provide only a limited window of growth for the crystalline films and typically are unable to provide pure or near pure cubic phase boron nitride films of high quality and having sufficiently large grain sizes desired for many applications for such films.
It therefore can be seen that a need exists for a method of synthesizing cubic boron nitride films that enables high growth rates and the formation of cubic phase boron nitride films of high purity, quality and larger grain sizes rapidly and economically without potentially damaging the films during formation due to a resputtering effect of the ion bombardment against the films.